1. Field of the Invention
The present invention relates to a digital broadcasting receiver tuner, and more particularly, to a double-conversion type digital broadcasting receiver tuner that is dedicated to receiving digital television broadcast, and performs high-performance digital demodulation with a reduced error rate.
2. Description of the Related Art
With advances in multi-media-related technology, analog broadcasting that uses analog signals as baseband signals is rapidly going to be replaced with digital broadcasting employing digital signals in the field of television broadcasting including ground television broadcasting, CATV (community antenna television) broadcasting, BSTV (broadcasting satellite television) broadcasting and the like.
In the television broadcasting, digital broadcasting is different from analog broadcasting not only in the signal format of the baseband signals but also in type of modulation. For example, digital broadcasting employs QPSK modulation or QAM modulation. For this reason, the television tuner for receiving both digital broadcast signal and analogy broadcast signal contains an analog signal demodulator circuit and a digital signal demodulator circuit to demodulate both the received analogy signal and digital signal, and the television tuner is thus a double-conversion television tuner.
FIG. 5 is a block diagram of an example of such a digital broadcasting receiver tuner, which is already disclosed in Japanese Patent Application No. 7-246323 filed by the applicant of this invention.
As shown in FIG. 5, a digital broadcasting receiver tuner 51 comprises an antenna input node 52, a wideband bandpass filter 53, a first frequency conversion stage 54, a second frequency conversion stage 55, a distributor 56, an analog signal demodulator 57, a digital signal demodulator 58, an analog signal output node 59, a digital signal output node 60 and a selector 61.
The first frequency conversion stage 54 comprises a first frequency mixer 62, a first local oscillator 63, a first phase-locked loop (PLL) 64 and a first intermediate frequency signal selecting bandpass filter 65, and the second frequency conversion stage 55 comprises a second frequency mixer 66, a second local oscillator 67, a second phase-locked loop (PLL) 68, and a second intermediate frequency selecting lowpass filter 69.
In the first frequency conversion stage 54, the first frequency mixer 62 is configured with its first input node connected to the antenna input node 52 via the wideband bandpass filter 53, with its second input node connected to an output node of the first local oscillator 63, and with its output node connected to a first input node of the second frequency mixer 66 in the second frequency conversion stage 55 via the bandpass filter 65. The first phase-locked loop 64 is configured with its input node connected to an output node of the first local oscillator 63, with its output node connected to a control input node of the first local oscillator 63, and with its control input node connected to an output node of the selector 61.
In the second frequency conversion stage 55, the second frequency mixer 66 is configured with its second input node connected to an output node of the second local oscillator 67 and with its output node connected to an input node of the distributor 56 via the second intermediate frequency selecting lowpass filter 69. The second phase-locked loop 68 is configured with its input node connected to an output node of the second local oscillator 67, with its output node connected to a control input node of the second local oscillator 67, and with its control input node connected to an output node of the selector 61. The analog signal demodulator 57 is configured with its input node connected to a first output node of the distributor 56 and with its output node connected to the analog signal output node 59.
The digital signal demodulator 58 is configured with its input node connected to a second output node of the distributor 56 and with its output node connected to the digital signal output node 60. In response to the control signal supplied by the second phase-locked loop 68, the second local oscillator 67 is switched so that it produces a second local oscillation signal at a first frequency when the baseband signal of the selected received signal is an analog signal, and so that it produces a second local oscillation signal of at a second frequency higher than the first frequency when the baseband signal of the selected received signal is a digital signal.
The digital broadcasting receiver tuner 51 generally operates as follows.
By operating the selector 61, a desired television broadcast signal is selected, and the baseband signal format of the selected broadcast signal is also selected. In the first frequency conversion stage 54, the output voltage of the first phase-locked loop 64 is changed in response to the selected broadcast signal, and the output voltage is used to control the frequency of the first local oscillation signal from the frequency-controlled, first local oscillator 63. In the second frequency conversion stage 55, the output voltage of the second phase-locked loop 68 is adjusted in response to the selected signal format, and the output voltage of the second phase-locked loop 68 is used to control the frequency of the second local oscillation signal from the frequency-controlled, second local oscillator 67, wherein the frequency of the second local oscillation signal is controlled to a first frequency when the baseband signal format of the selected broadcast signal is an analog signal, and is controlled to a second frequency higher than the first frequency when the baseband signal format of the selected broadcast signal is a digital signal.
When the broadcast signal received at an antenna comes into the antenna input node 52, the wideband bandpass filter 53 filters out unwanted components of the received signal, and the filtered signal is supplied to the first frequency conversion stage 54. The first frequency conversion stage 54 frequency-mixes the received signal and the first local oscillation signal, and feeds the resulting frequency mixed output to the first intermediate frequency signal selecting bandpass filter 65. The first intermediate frequency signal selecting bandpass filter 65 extracts the received signal selected by the selector 61 out of the remaining received signals, namely, extracts the received signal of a first intermediate frequency f.sub.IF1 as a first intermediate frequency signal and then feeds it to the second frequency conversion stage 55.
The second frequency conversion stage 55 frequency-mixes the first intermediate frequency signal and the second local oscillation frequency signal of the first frequency or the second frequency produced by the second local oscillator 67, and feeds the resulting frequency mixed output to the second intermediate frequency selecting lowpass filter 69. The second intermediate frequency selecting lowpass filter 69 extracts frequency-converted signal of the second intermediate frequency f.sub.IF2A or f.sub.IF2D as the second intermediate frequency signal and feeds it to the distributor 56.
The distributor 56 distributes the supplied second intermediate frequency signal to both the analog signal demodulator 57 and the digital signal demodulator 58. The analog signal demodulator 57 analog demodulates the second intermediate frequency signal when the baseband signal format of the second intermediate frequency signal supplied is an analogy signal, and outputs the demodulated signal at the analog signal output node 59. The digital signal demodulator 58 digital-demodulates the second intermediate frequency signal when the baseband signal format of the second intermediate frequency signal supplied is a digital signal, and outputs the demodulated signal at the digital signal output node 60.
As analog broadcasting today is expected to be rapidly replaced with digital broadcasting in television system, the demand on the television tuner is shifting from a television tuner provided with both an analog signal demodulator circuit and a digital signal demodulator circuit for receiving selectively analog broadcasting and digital broadcasting to a digital television broadcasting receiver tuner provided with a digital signal demodulator circuit only, dedicated to receiving digital broadcasting.
When the digital broadcasting receiver tuner dedicated to receiving digital broadcasting is of such a double conversion type digital broadcasting receiver tuner as the already described digital broadcasting receiver tuner 51, a unique problem associated with reception of the digital broadcast signal need to be solved.
More particularly, when the second local oscillation signal from the second oscillator is fed to the second frequency mixer in the second frequency conversion stage, the second local oscillation signal may appear at the first intermediate frequency signal input node of the second frequency mixer without being sufficiently attenuated through the second frequency mixer. The second local oscillation signal propagates backward through the bandpass filter, and reaches the first frequency mixer in the first frequency conversion stage where it is mixed with the first local oscillation signal output from the first local oscillator, and thus spurious component having the same frequency as the second intermediate frequency signal from the second frequency mixer is generated. The spurious component strays through the bandpass filter to the second frequency mixer, and as a result, the spurious component may be superimposed onto the second intermediate frequency signal. Furthermore the spurious component may stray through a power supply circuit to the second frequency mixer, and may be superimposed to the second intermediate frequency signal output from the second frequency mixer. The spurious component superimposed is of the same frequency as the second intermediate frequency signal while their phases are different, and thus performance in the demodulation of QPSK or QAM modulated digital signal will be degraded.
Although the above-described digital broadcasting receiver tuner 51 may be easily modified to a receiver tuner dedicated to receiving digital broadcasting by eliminating the analog signal demodulator circuit and its associated circuit, no consideration is given to resolving the unique problem associated with reception of the digital broadcast signal, and thus high performance is not expected in the demodulation of the digital signal.